A group of influential US-based activists has said that South Sudan, which celebrated the second anniversary of its independence on Tuesday, faces an “increasingly perilous fate” and is sliding towards “instability, conflict and a protracted government crisis” due in part to rampant official corruption and abuse of power.

The warning, in an open letter to South Sudan's president, Salva Kiir, and his deputy, Riek Machar, comes amid rising alarm about ethnic violence and alleged atrocities in eastern Jonglei state.

Hilde Johnson, the UN special envoy to South Sudan, told the security council this week that over-stretched peacekeepers needed surveillance drones and helicopter gunships to deal with the crisis.

The group of activists, known as the Friends of South Sudan, includes Roger Winter, a former US state department special envoy, John Prendergast, co-founder of the Enough Project, and Professor Eric Reeves, a noted Sudan scholar.

The group became known as the “midwives” for their successful lobbying of the Bush administration and the UN in support of an independent South Sudan and for their backing for the 2005 comprehensive peace agreement, which ended the civil war.

In their letter they assure Kiir of their goodwill, then launch into a fierce attack on almost every aspect of his government's record.

“Over the past several years—but the last six months in particular—South Sudan government security forces have engaged in a campaign of violence again civilians simply because they belonged to a different ethnic group or they are viewed as opponents of the current government,” the letter states.

“This violence is shocking and has included rape, murder, theft and destruction of property. We are particularly concerned about the evidence emerging of abuses by government forces in Jonglei. These terrible crimes occur because government forces believe they have the power to act with impunity.”

The group says reforms are urgently needed to tackle widespread human rights abuses in South Sudan, including extra-judicial killings, arbitrary arrests, media intimidation and corruption. Failing this, it suggests, the infant state may collapse.

“After almost nine years of self-rule, the government is still failing to meet the basic needs of its people . . . Those who have benefitted—who have become wealthy by misappropriating government funds—have often sent their families outside South Sudan, their children to private schools abroad, and have obtained the best medical services available in the world.

“This occurs while ordinary citizens who remain in South Sudan cannot afford even basic health services or modest educations for their children.” Despite Kiir's admission that corruption was a serious problem, little had been done to curb it, the letter said.

Recent assessments by the UN and NGO groups based in Juba, South Sudan's capital, say the Jonglei crisis is growing especially urgent, with an estimated 120,000 people affected, and refugee and internally displaced numbers increasing.

Hospitals run by the medical charities Medecins Sans Frontieres (MSF) and Merlin in the towns of Pibor and Boma were looted in May and these and other population centres are now reportedly deserted.

Violence has been on the rise since March, when the army launched a new offensive against rebels, led by David Yau Yau, opposed to Kiir's ruling party, the Sudan People's Liberation Movement (SPLM). Yau's militia is predominantly Murle, a marginalised, minority tribe with a long history of conflict with the Lou Nuer, who form the bulk of the army rank-and-file.

Evidence of the deliberate targeting of civilians by government forces or their militia allies is mounting. In a joint statement on Jonglei dated 20 June, the UN and NGOs detailed reports of the alleged killing by the army of 16 civilians in Pibor and Boma and the execution of 12 Murle outside an army camp. Incidents or rape and sexual abuse of women and girls were also increasing.

The UN said last month it had received $5.4m (GBP3,6m) from its central emergency response fund to buy two helicopters and medical equipment to treat or evacuate civilians in Jonglei. But the onset of the rainy season is further hindering humanitarian access.

MSF's head of mission in South Sudan, Vikki Stienen, said the UN money would achieve little unless civilians were able to return to Pibor and Boma to receive help.

“There are still some 120,000 people missing. They are trapped out in the swamps, and it is getting wetter and wetter without a way now for us to access them.”

By arrangement with the Guardian